Scientific Method —

Ozone chemistry confounds everyone

New research throws our understanding of the chemistry driving ozone depletion …

The ozone hole and depletion of the ozone layer is old news. We have had hard data on the layer for a more than 20 years and that data tells us, unequivocally, that the ozone layer is disappearing at a rate of about three percent per year. The process that drives this has also not been subject to much attention for sometime: we thought we understood it very well. In a series of four reaction steps, an unstable oxidated chlorine dimer (ClO) combines with a second dimer—a reaction that is mediated by collision with a third party molecule—to form Cl2O2. One of the chlorine molecules is split off by a high energy photon and the second chlorine is split off through collisions with other molecules. The two free chlorine atoms then react with ozone molecules to recreate the two dimers and over the whole cycle, converts two ozone molecules (O3) into three oxygen molecules (O2).

The rates of each step needed to be determined before it could be concluded that chlorine based molecules have been responsible for the depletion of the ozone layer, and chemists thought they had that nailed. The weak link in our knowledge of the reaction rates has always been the light initiated cleaving of chlorine. Although various labs had attempted to measure this step, the reported reaction rates (more precisely, the absorption cross section of Cl2O2) varied by a factor of five, leaving this step poorly understood. It appears that atmospheric chemists weren't too concerned by this because even the lowest reported value was fast enough. Models based on this reaction chain, combined with a few other, less significant paths, accounted for the observed ozone depletion. Case closed, right?

Wrong, apparently. Concerned over the wide distribution of values and the complacency of scientists, a crank, working in his garage, has managed to overturn 20 years of dogma in a blindingly simple experiment. Umm, no, that isn't correct either. In fact, a team of scientists from the Jet Propulsion Laboratory have put together a rather complicated experiment, one that allowed them to isolate the Cl2O2 molecule in a form that was much more pure than what had been previously obtained. They did this through a combination of laser induced reactions, cooling, and trapping. Having obtained a much more pure form, the researchers were able to use UV lasers to cleave the chlorine molecule and measure the rate of the subsequent reactions. To their shock, they found that the reaction rate was not just at the lower end of the published results, but about an order of magnitude slower than the average of previously reported values.

Although this work needs to be replicated, it may have far-reaching consequences in both atmospheric physics and politics. Firstly, using the new reaction rate, scientists can no longer account for 60 percent of the observed ozone depletion. Although it is still thought that chlorine based catalytic reactions are the major cause of ozone depletion, we no longer have a strong link between theory, experiment, and observation. This may give policy makers all the excuse they need to begin (or continue) ignoring the Montreal convention. Most importantly, scientists had, based on chlorofluorocarbon emissions and the chemical reaction pathways, predicted a slow recovery of the ozone layer. We can no longer be confident about that either. Lastly, and this sounds like a bit of a stretch, we can't predict how ozone chemistry and global climate change will interact. Basically, all these reactions are temperature dependent and are probably dependent on light from the sun. Previously, we thought the influence of climate change on ozone chemistry would be minor. Now the honest answer is that we don't know.

I will finish by explaining the line about a crank in his garage overturning scientific dogma. All of my life, I have been exposed to people who believe that scientists are more attached to their theories than they are to data. They think that scientists, as a group, persecute and suppress anyone who tries to demonstrate that the existing theory is in any way flawed. This is especially true when science connects with politics and/or our beliefs. I cannot think of a finer immediate example to present as a counterpoint.

Journal of Physical Chemistry, 2007, DOI: 10.1021/jp067660w

Chris Lee
Chris writes for Ars Technica's science section. A physicist by day and science writer by night, he specializes in quantum physics and optics. He lives and works in Eindhoven, the Netherlands. Emailchris.lee@arstechnica.com//Twitter@exMamaku